This study examined how free-floating macrophyte cover (principally composed of duckweeds [Lemna spp.]) and prevalence of floating filamentous algal mats (metaphyton) varied within and among lakes within three reaches of the Upper Mississippi River. Data were collected using standard sampling approaches over the period 1998 to 2008. Duckweed cover varied primarily within and among lakes; in comparison filamentous algae prevalence varied primarily among lakes and lake-years. Duckweed cover increased with submersed aquatic vegetation (SAV) abundance at within-lake and among-lake-year scales; in comparison, filamentous algae prevalence increased with SAV abundance at within-lake, among-lake and year scales. Given adjustment for SAV, filamentous algae prevalence decreased with increasing lake connectivity but was not statistically associated with annual changes in mean river discharge; duckweed cover was not associated with either connectivity or discharge. Documenting the relatively high levels of variation within lakes and of year-to-year variation in lake means improves our understanding of the dynamic nature of aquatic plant and algal communities in the Upper Mississippi River and will assist efforts to manage or control aquatic plants and nuisance algae in this region. In particular, this work explicitly characterizes sources of variability in free-floating macrophyte cover and filamentous algae prevalence, and highlights how this variation may complicate efforts to evaluate the short-term success of management and control efforts.
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The relationship between host density and parasite transmission is central to the effectiveness of many disease management strategies. Few studies, however, have empirically estimated this relationship particularly in large mammals. We applied hierarchical Bayesian methods to a 19-year dataset of over 6400 brucellosis tests of adult female elk (Cervus elaphus) in northwestern Wyoming. Management captures that occurred from January to March were over two times more likely to be seropositive than hunted elk that were killed in September to December, while accounting for site and year effects. Areas with supplemental feeding grounds for elk had higher seroprevalence in 1991 than other regions, but by 2009 many areas distant from the feeding grounds were of comparable seroprevalence. The increases in brucellosis seroprevalence were correlated with elk densities at the elk management unit, or hunt area, scale (mean 2070 km2; range = [95-10237]). The data, however, could not differentiate among linear and non-linear effects of host density. Therefore, control efforts that focus on reducing elk densities at a broad spatial scale were only weakly supported. Additional research on how a few, large groups within a region may be driving disease dynamics is needed for more targeted and effective management interventions. Brucellosis appears to be expanding its range into new regions and elk populations, which is likely to further complicate the United States brucellosis eradication program. This study is an example of how the dynamics of host populations can affect their ability to serve as disease reservoirs.
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